Retarding field electron-optical apparatus

ABSTRACT

A method of noncontact testing and an electron optical system includes an electron source for producing a high energy electron beam, a retarding field objective lens system for receiving and focussing the high energy electron beam to produce a focussed low energy electron beam, and a magnetic deflector for deflecting the focussed low energy electron beam to the sample, thereby to expose the sample to the low energy electron beam, and simultaneously maintaining a predetermined spot size of the beam. The retarding field objective lens system includes a device for retarding electrons in the low energy electron beam directed to the sample and for accelerating electrons, emitted by the sample upon being irradiated by the low energy electron beam.

This is a Continuation of Application Ser. No. 08/329,033 filed Oct. 25,1994 now abandoned.

DESCRIPTION BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved retarding field opticalapparatus for focusing and deflection of high resolution, high-current,low-energy charged particle beams. Additionally, the present inventionrelates to apparatus for detecting the charging currents produced bysuch a beam when the beam interacts with a target, the detectionapparatus being physically and electrically integrated with theretarding field apparatus so as to provide a system providing excellentsignal detection and improved optical performance.

More particularly, a preferred embodiment of the present inventionrelates to a method and system for using an electron beam to testconductive networks in multi-chip modules (MCMs) for such defects asopens between networks which should be electrically connected and shortsbetween networks which should be electrically isolated, using anelectron beam capable of large field deflections at low voltages.

2. Description of the Related Art

There are many important applications for high-resolution, high-current,low-voltage charged particle beams including, for example, applicationsin microscopy, surface analysis, integrated circuit testing, andlithography.

Within the field of electron beam column technology, which is concernedwith apparatus for the production, focussing, and deflection of beams ofelectrons or other charged particles, the difficulty of producinglow-voltage beams by conventional techniques is well known. Factorscontributing to these difficulties include: i) the proportionality ofsource brightness to beam voltage; ii) the additional reduction ofsource brightness at low-voltage due to electron-electron interactioneffects and space charge saturation effects; iii) the increasedchromatic aberration of lenses and deflectors; and iv) increased beamsensitivity to the disturbing effects of ambient magnetic fields andcharging within the column.

Recently, the advantages of employing an unconventional approach to theproduction of low voltage beams have been recognized. The new approachinvolves accelerating the beam to a kinetic energy that exceeds thedesired landing energy at the target and subsequently decelerating orretarding the beam to the desired impact kinetic energy. This newapproach is termed "a retarding field technique" (RFT).

The RFT has two primary advantages. First, the stronger acceleratingfield at the electron source improves source brightness at low voltage.Secondly, the decelerating or retarding field, appropriately designed,can be combined with a conventional lens with the result that theperformance of the combination is much better than that of theconventional lens alone.

The optical performance advantages of the RFT are only fully realizedwhen the retarding field immediately precedes the target. Such aconfiguration, consists of a conventional lens (either magnetic orelectrostatic), a retarding field (electrostatic) and then the target.For a probe forming system, the configuration normally must also includemeans for deflecting the beam. Such a configuration, includingdeflection means, is termed "a retarding field objective" (RFO).

While the optical performance advantages of RFOs are well known, aproblem remains which limits their usefulness. In most applications, oneis interested not in the primary beam itself, but in its interactionwith a target. Unfortunately combining RFOs with conventional chargedparticle detectors, which collect electrons emitted because of thebeam-sample interaction, is difficult.

The problem arises since the retarding field, which decelerates theprimary electrons, is an accelerating field for the electrons emittedfrom the sample. This acceleration confines the emitted electrons totrajectories which are physically close to the primary beam trajectory.The narrow emission angle of the emitted electrons and their proximityto the primary beam electrons make their detection difficult, since thedetector must be simultaneously transparent to primary beam electronsand somehow collect emitted electrons. Further, the collector must bedesigned so as not to adversely affect the electrostatic field withinthe retarding field lens.

As a result, as a practical matter, the detector must be outside theretarding field, which means overall system length must be increased toaccommodate it. Increased system length decreases the opticalperformance.

Thus, the retarding field allows production of a better primary beam butsimultaneously limits or compromises the ability to determine thesample's features with this beam using conventional electron detectors.

Heretofore the present invention, therefore the design of retardingfield objective lenses was driven by a compromise between goodperformance for primary beam production and good performance for emittedelectron detection.

These compromises are well illustrated in a conventional design in whichto separate the incoming primary beam electrons and the outgoing emittedelectrons, an additional optical element is required. More specifically,a bending magnet, which directs the emitted electrons away from theprimary beam so that they can be detected by conventional detectorsphysically distant from the primary beam, has been required.

This approach has several problems. In addition to the complexity andexpense of the bending magnet, there are two additional problems.

First, the bending magnet degrades primary beam resolution. Secondly,the bending magnet rigidly constrains the design of the beam deflectionsystem. The deflection system must not only deflect the primary beam onthe target, but simultaneously deflect the emitted electrons so thatthey travel backwards along the axis of the system so as to enter thebending magnet. This constraint on the deflection system limits theoptical performance by restricting the deflection elements toelectrostatic elements and limiting how they may be positioned andenergized.

Further, it is well known that contactless test systems are needed forthe substrates of multi-chip-modules. In the production of multi-chipmodules (MCMs), the component parts of the modules should be tested fordefects before assembly to minimize the cost of repairing such defectsand to maximize the yield of operable devices.

Component testing includes testing for and the detection of opens andshorts in the conductive networks of the substrate on which theintegrated circuits are mounted. Mechanical probe systems are commonlyused for this purpose, but these systems oftentimes damage substrates,add particulate contamination, have limited throughput, and are notapplicable to feature sizes below approximately 25 micrometers, or tosmall features recessed into insulators. Because of these problems,non-contact techniques have been investigated for open and short defectdetection.

Among the non-contact test systems are electron beam systems which usevoltage contrast. However, voltage contrast systems have severaldrawbacks including the propensity to detect "false" shorts, i.e., todetect shorts between nets which have an inter-net resistance of100megohms or more.

A second drawback of voltage contrast systems is that primary beamdeflection must cover the entire substrate to obtain practical testtimes.

In view of the foregoing problems of the conventional systems, there isa need for a non-contact substrate testing system which overcomes theabove-mentioned problems.

A test system and method which overcomes the above-mentioned problemshave been described in copending U.S. Patent Application Ser. No.08/036,781, to Golladay, filed on Mar. 25, 1993, now U.S. Pat. No.5,404,116, incorporated herein by reference and assigned to one of theco-assignees of the present application.

The method of Golladay '781 uses an induced current signal to detectdefects, and is referred to as an induced current test method (ICTM) inwhich an induced current signal is detected to determine whether defectsexist. The advantages of ICTM include: i) its reduced propensity todetect false shorts; ii) the elimination of the need for full substratedeflection; iii) minimal insulator charging effects because of the useof a relatively low primary beam landing energy, eV_(L) (e.g., typicallyin the range of 500-800 eV) on the substrate; and iv) its ability tomeasure network capacitance.

However, while many of the advantages of the ICTM result from using alow voltage beam, the production and deflection of low voltage beamswith high beam current and small spot size is difficult, as discussedabove. Nevertheless, despite these difficulties, obtaining sufficientlyhigh beam current in a sufficiently small focussed probe which can bedeflected electromagnetically over a sufficiently large deflection fieldis essential to achieving optimum tester performance.

Thus, the conventional systems, i.e., those not employing RFT, havesuffered from not being able to produce a high-resolution, low energybeam with a sufficiently high beam current, nor to deflect a low energybeam over a sufficiently large area, for high throughput productiontesting due to inherent problems with the conventional approaches.

Moreover, existing systems employing RFT have suffered from the samelimitations due to design compromises arising from the incompatibilitiesof probe formation and emitted electron detection in those systems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a systemand method for overcoming the above problems of the conventional systemsand techniques.

Another object of the invention is to provide a retarding fieldobjective lens and magnetic deflection system to produce a small, highcurrent probe, and to provide means for deflecting the probe over alarge field while maintaining acceptable spot size.

It is yet a further object of this invention to provide means fordetecting the induced current signal from a substrate at elevatedpotential by an induced current detector which may be referenced to thesubstrate potential.

Yet another object is to provide a completely non-contact testing methodand system that avoids the problems of contact damage and contaminationthat are inherent in contact testing, and to increase the testing speedusing the electron beam as opposed to a mechanically movable probe.

It is a further object of this invention to provide a retarding fieldwhich is compatible with a planar extraction grid provided above thesubstrate being tested, and which ensures that electrons emitted fromthe substrate that pass through the extraction grid do not return to thesubstrate.

In one aspect of the invention, a method for achieving the above andother objects is provided comprising:

elevating the potential of an electrical device; charging saidelectrical device with a low energy electron beam; producing a focussedlow energy electron beam; deflecting the low energy electron beam topredetermined portions of the electrical device, thereby to expose thepredetermined portions to the low energy electron beam, andsimultaneously maintaining a predetermined spot size of the beam, aninduced current signal being produced when the predetermined portionsare exposed to the focussed electron beam; and detecting the inducedcurrent signal to determine characteristics of the electrical device.

In another aspect of the invention, a system for achieving the above andother objects is provided including:

means for elevating the potential of an electrical device; means forproviding a low energy electron beam to the electrical device; means forproducing a focussed low energy electron beam; means for deflecting thelow energy electron beam to predetermined portions of the electricaldevice, thereby to expose the predetermined portions to the low energyelectron beam, and simultaneously maintaining a predetermined spot sizeof the beam, an induced current signal being produced when thepredetermined portions are exposed to the focussed electron beam; andinduced current detecting means for detecting the induced current signalto determine characteristics of the electrical device.

With the invention, a retarding field objective lens and a deflectionand detection system for electron beam non-contact testing of MCMs forsuch defects as opens and shorts, are provided which use an electronbeam capable of large field deflections at low voltages to avoidcharging problems with the MCM.

With the invention, wiring networks within substrates used tointerconnect integrated circuits (ICs) can be electrically tested in acost-effective manner. Detection of open and short defects in thesubstrates is accomplished with the invention by an electron beam systemutilizing an induced current test method and a retarding field objectivelens and deflection system which produces and deflects a highresolution, high current, but low energy, primary electron beam, whilesimultaneously providing for induced current signal detection.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of preferredembodiments of the invention with reference to the drawings, in which:

FIG. 1(a) illustrates the electrostatic potential on the axis of anelectron beam column in a conventional column 101 and for a columnincorporating retarding field technology in the form of a retardingfield objective lens 102;

FIG. 1(b) illustrates the kinetic energy of the primary electron beam onthe axis of an electron beam column for a conventional column 103 andfor a column incorporating a retarding field objective lens 104;

FIG. 2 illustrates a conventional retarding field objective lens,deflection system and emitted electron detector;

FIG. 3 illustrates one embodiment of the retarding field objective lens,deflection system and electron detector according to the presentinvention;

FIG. 4 illustrates a substrate tester designed for the practice of theinduced current test method (ICTM) but with conventional columntechnology;

FIG. 5 is a cross-sectional view of a second embodiment of the presentinvention including a magnetic lens, upper and lower deflection yokes, aretarding electrode assembly, and an induced current detector;

FIG. 6 illustrates possible electrostatic potential distributions as afunction of position within the retarding field in which curve A shows afield which is unacceptable because too many emitted electrons wouldreturn to the substrate, i.e., the field is not extractive, curve Bshows a field which is extractive but still unacceptable because thefield is too strong at the extraction grid, and curve C illustrates afield which is acceptable in terms of being both extractive and weak atthe extraction grid;

FIG. 7 is a cross-sectional view of the retarding electrodes of theinvention, and also shows schematically the trajectories of emittedelectrons;

FIG. 8(a) illustrates the minimum voltage (e.g., V3) needed on anelectrode (e.g., a third electrode) used by the inventive system, for SEextraction at the field corners as a function of the voltage (e.g., V2)on another electrode (e.g., a second electrode) of the inventive system;

FIG. 8(b) illustrates the electrostatic field above an extraction gridin the center and field corners as a function of V2, V3 chosen from thecurve of FIG. 8(a) in which weak fields are obtainable for -13.5kV<V2<-12.0 kV;

FIG. 9 illustrates the electrostatic potential as a function of axialposition at r=0 and r=23 mm for a selected value of V2 and V3, (e.g.,V2=-12.8 kV, V3=-2.257 kV);

FIG. 10 illustrates the deflected beam trajectory within the deflectionyokes and the retarding electrostatic field, with the distance from thebeam to the optical axis being plotted as a function of axial position;

FIG. 11 shows the lens properties as a function of the lower yokeorientation for the case of a strength ratio of the lower yoke/upperyoke of 3.0; and

FIG. 12 illustrates the calculated first and third order lens propertiesof the retarding field objective lens according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Prior to a description of the structure and method of the invention,basic principles of the theoretical construct of the invention will bediscussed and, more specifically, a method having a retarding fieldtechnique (RFT) which is used in the present invention will bediscussed.

Hitherto the invention, a retarding field technique (RFT) for producinglow energy beams has been advocated. For purposes of this application, a"low" energy beam is a beam having an energy of < 1 kV, whereas a "high"energy beam is one having an energy > 1 kV and more typically > 5 kV.While having limited applicability to planar or nearly planar targets,the RFT provides for higher beam currents in a given probe size comparedto the conventional methods. RFT improves overall system performance byincreasing source brightness and by reducing the aberrations of theprobe forming- and beam deflection optical elements. For example, aten-fold increase in beam current on the lens axis has been found withthe retarding field approach.

However, prior to the invention, retarding field systems for large fielddeflection (>10×10 mm) have been unknown.

The difference between the RFT and the conventional approach can beunderstood by considering the electrostatic potential along the axis ofan electron beam column, and the kinetic energy of the primary beamelectrons. The potential and the kinetic energy of the primary beamelectrons are illustrated for a conventional system and a systememploying RFT in FIGS. 1(a) and 1(b), respectively.

Typically, for both safety and convenience, the anode of the electronsource and the bulk of the column are held at ground potential, (e.g.,V_(col) =0 V (Volts)), for both the conventional and retarding fieldapproaches.

Moreover, with the conventional approach, the target is also at groundpotential, while the electron source, or cathode, is at a negativepotential, V_(cath), corresponding to the desired beam landing kineticenergy, E_(L). For example, if E_(L) =600 eV (electron volts), V_(cath)would be -600 V. The primary beam electrons are accelerated to theirlanding kinetic energy in the vicinity of the cathode, and retain thatkinetic energy until impacting the substrate.

The retarding field approach involves producing a beam with a higherkinetic energy than the landing energy and then decelerating the beamwith electrostatic fields to achieve a relatively low landing energy.Normally, to reduce saturation and interaction effects in the electrongun, V_(cath) is chosen to be at least 5.0 kV negative relative to thebulk of the column which is typically at ground potential. The target(e.g., sample or substrate) potential is also elevated to negative highvoltage. The potential V_(sub), is chosen so that it satisfies therelation of:

    E.sub.L =-(V.sub.cath -V.sub.sub)

In the above equation, E_(L) is in units of electron volts and thevoltages are in units of volts. So, for example, if V_(cath) =-5 kV, andE_(L) =600 eV, then V_(sub) =-4.4 kV.

Thus, the primary beam decelerates in the retarding field in front ofthe sample, thereby reducing the beam landing kinetic energy to 600 eV,although the kinetic energy of the beam is at 5.0 keV throughout most ofthe column. For simplicity and convenience, the present invention isdescribed assuming V_(col) =0 Volts and the above values for V_(cath),E_(L) and V_(sub), although these are only typical or nominal values andany values may be selected, as appropriate, by the ordinarily skilledartisan in the purview of the present application. However, it isrequired that V_(cath) <V_(sub) <V_(col).

The electrostatic potential and beam kinetic energy along the columnaxis of the conventional approach and the RFT approach are illustratedin FIGS. 1(a) and 1(b), respectively. FIG. 1(a) shows the electrostaticpotential along the axis of the column from the electron source (orcathode) to the target, illustrating how with the RFT the cathodepotential is made more negative, and the target potential is alsoelevated to negative potential. Curve 101 illustrates the conventionalsystem whereas curve 102 illustrates the RFT.

FIG. 1(b) illustrates the beam kinetic energy of the two approaches.With RFT (as shown by curve 104), the beam kinetic energy is relativelyhigher over most of the column length declining to the landing energy inthe vicinity of the target. Curve 103 illustrates the conventionalapproach.

Along with improving source brightness and reducing beam sensitivity tocharging and magnetic field interference, the retarding field approachhas another important advantage. That is, a properly designedelectrostatic retarding field can operate in combination with aconventional (non-retarding) lens, to produce a high performance lenswith low aberration coefficients.

Two factors which are important to good performance in focusing anddeflecting the primary beam are the form of the axial potentialdistribution in the retarding section, and the trajectory of thedeflected electrons in the retarding field. The axial potentialdistribution is optimized by appropriate choice of the geometry and biaspotentials of the retarding electrodes.

The trajectory of the deflected electrons is optimized by the choice ofdeflector type (electrostatic or magnetic), the number, axial position,and relative angular orientation of the deflectors and the relativestrengths of the electrical signals which drive them.

However, despite the advantages of retarding field optics for theprimary beam, as discussed above, there are problems related tocombining RFT with conventional electron detectors. The problems arisesince the retarding field which decelerates the primary electrons is anaccelerating field for the electrons emitted from the sample. Thisacceleration confines the emitted electrons to trajectories which arephysically close to the primary beam trajectory. The narrow emissionangle of the emitted electrons and their proximity to the primary beamelectrons makes their detection difficult, since the detector must besimultaneously transparent to primary beam electrons and somehow collectemitted electrons.

These difficulties are illustrated in a conventional design which isillustrated in FIG. 2. This conventional system 20 includes aconventional magnetic lens represented schematically by its pole pieces21, an upper electrostatic deflector 22, a lower electrostatic deflector23, retarding field electrodes 24, a bending magnet 25 and electrondetector 26.

The bending magnet 25 is for separating the primary beam 27 travelingtoward the sample from the emitted electron beam traveling back up thecolumn. The bending magnet 25 directs the emitted electrons 28 away fromthe primary beam 27 so that they can be detected by conventionaldetectors 26 physically distant from the primary beam 27.

The above-mentioned conventional approach has several problems. Inaddition to the complexity and expense of the bending magnet, there aretwo additional problems. The first problem is that the bending magnetdegrades primary beam resolution, and the second problem is that usingthe bending magnet rigidly constrains the design of the beam deflectionsystem. The deflection system must not only deflect the primary beam onthe target, but at the same time deflect the emitted electrons so thatthey travel backwards along the axis of the system so as to enter thebending magnet. This constraint on the deflection system limits theoptical performance by requiring electrostatic deflection elements andlimiting how they may be positioned and energized.

The conventional deflection system consists of a pair of identicalelectrostatic deflectors, an upper deflector 22 and a lower deflector23, equally and oppositely driven. Each deflector typically consists ofelements for producing deflections in two orthogonal directions,conventionally designated "x" and "y" deflectors. Such a pair ofidentical deflectors equally and oppositely driven displaces a beamparallel to itself. Thus, the primary beam initially traveling on theaxis of the system 20 is displaced off this axis but still travelsparallel to it. The emitted electrons travel the same path in thereverse direction, thereby ending up traveling along the system axis tothe bending magnet 25.

This approach has the drawback that it can be implemented only withelectrostatic deflectors. Magnetic deflectors are excluded because themagnetic deflection is oppositely directed for the incoming and outgoingbeam. This effect is exploited by the conventional system in the bendingmagnet 25 to separate the incoming and outgoing beams. Magneticdeflectors would have the same beam separating effect and thereforewould not direct the emitted electrons back along the optical axis.

The requirement to use electrostatic deflectors is a significantlimitation because magnetic deflectors offer improved opticalperformance and simpler practical implementation. The improved opticalperformance results from i) inherently lower chromatic aberrations, ii)and the deflection yokes being positionable so that the magneticdeflection fields overlap the electrostatic retarding fields, therebyshortening system length.

There are also several practical advantages with respect to ease ofimplementation of magnetic yokes. For example, the ability to positionthe magnetic deflectors outside the vacuum vessel means that the yokeposition or orientation can be more easily adjusted, the system is moreeasily assembled and serviced, and there is no possibility ofbeam-induced contamination of the deflectors.

Furthermore, the conventional system's requirement of equal and oppositedeflection sensitivity for the upper and lower deflectors is asignificant limitation. The benefits of a more general approach to beamdeflection are well known. With a deflection system consisting ofmultiple yokes, improved performance has been demonstrated by optimizingthe yokes' geometry as well as their relative strengths andorientations, their positions relative to each other and relative to amagnetic objective lens.

The problems and design restrictions of the conventional approachdiscussed above are a direct result of the difficulties of combiningretarding field optics and conventional emitted electron detectors.

These problems and design restrictions are eliminated with the presentinvention in which the retarding field configuration includes magneticdeflectors and is used in combination with an alternative signaldetection scheme. Of primary interest for one preferred embodiment ofthe present invention is an induced current detector, but other types ofradiation detectors are also possible (e.g., X-ray, photo or infrareddetectors). Such a combination of technologies was unknown hitherto theinvention and its advantages were unrecognized.

An example of a first preferred embodiment of the present invention isshown in FIG. 3, which illustrates some of the general designpossibilities enabled by the present invention. A more detaileddescription of a second embodiment specifically directed toward theapplication of substrate testing will be given later.

In the design of FIG. 3, a plurality of electrodes 30 provide theelectrostatic retarding field. The electrodes 30 include a planarelectrode 30a below a target 31 which is at an elevated negativepotential. The electrodes above the target 31 are constructed to haveaxial symmetry and are positioned so that their symmetry axes arealigned with the axis of the rest of the electron optical column.

The magnetic focusing field is provided by the magnetic lens structure34 which includes a coil 35. Beam deflection is provided by at least onemagnetic 32 deflection element and/or electrostatic deflection element33. The electrostatic deflection means 33 are located above the retardelectrodes 30. The magnetic deflection elements 32 are located eitherabove the retard electrodes, or outside the retard electrodes, or belowthe target, or in any combination of the above positions.

Hereinafter, a detailed description is provided of a preferredembodiment of the present invention which is directed toward theapplication of substrate testing. For this application, the detectedsignal is the current, capacitively induced, in a conductive structurewithin or close to the target, when the target is irradiated by a lowvoltage primary beam.

Unlike a conventional electron detector, which collects emittedelectrons, the induced current detector senses the effect of the emittedelectrons without collecting them. Therefore, this detector isindifferent to the emitted electron trajectories as long as the emittedelectrons do not return to the substrate under test.

Furthermore, the induced current detector can be physically locatedbeneath the target. The detector's physical location and itsindifference to the emitted electron trajectories give the electronoptical designer a great deal of freedom to optimize those componentsabove the sample (e.g., namely the RFO and deflection system), withrespect to performance in focusing and deflecting the primary beam.

Referring now to the drawings, and more particularly to FIG. 4, there isshown a block diagram of a conventional electron-beam test system foruse with the present invention. A description of the system of FIG. 4 isfound in U.S. Patent Application Ser. No. 08/036,781, now U.S. Pat. No.5,404,110, mentioned above and incorporated herein by reference, and inwhich a conventional approach to producing a low energy beam wasfollowed.

With this approach, a cathode (e.g., an electron beam probe gun) 12 isbiased to a negative potential, corresponding to the desired beamlanding energy (e.g., -600 V potential for a 600 eV landing energy), andthe rest of the electron beam optical system including the targetsubstrate 36, a vacuum chamber 20, a conductive structure 59capacitively coupled to the target substrate, extraction grids 48, 49coupled to an extraction grid bias supply 50, and a current amplifier 65coupled to a reference bias supply 70, are each at ground or biased bylow voltage power supplies referenced to ground. The electron sourcebeam emanating from the electron beam probe gun 12 is demagnified by afocussing lens assembly 14 including an upper magnetic objective lens(unreferenced) and is focussed onto the substrate 36 by the magneticobjective lens. Beam deflection is performed by annular deflection yokes(coils) 16 driven by a deflection generator 18. The current amplifier 65supplies induced current in a conductive structure within or in closeproximity to the test specimen.

The system shown in FIG. 4 provides the platform for the retarding fieldoptics and detection scheme of the present invention. The high voltagesupply and electron beam source 12 support production, alignment, anddemagnification of a higher energy beam, 5 kV or more, for the presentembodiment of the invention. However, certain changes must be made inthe physical structure of a conventional system to accommodate thedeflection system and retard electrodes of the present invention.

The detailed physical design of this preferred embodiment of theinvention has been dictated in part by the desire to facilitateincorporation of the present invention into existing electron beamsystems with minimal modifications. To that end, a design was chosenwhich required minimal changes to a conventional magnetic lens, and theretarding field electrode assembly was designed to fit in the spacebetween the sample and the magnetic lens in the space usually occupiedby the emitted electron detectors.

The present invention, as shown in FIG. 5, includes the provision,positioning and implementation of a magnetic objective lens 500, adeflection system 501, retarding field electrodes 502, an inducedcurrent detector 503, a reference bias supply 507 for the currentamplifier for measuring the current induced during irradiation of atarget 504, and optionally one or more extraction grid(s) 505 and anextraction grid bias supply in combination. A suitable biasing device507, as is known in the art, is provided for the electrodes 502 so as tobias the electrodes to predetermined potentials. Device 507 can be thesame (or separate from) from the reference bias supply for the currentdetector 503 and a bias supply for grid 505.

Before describing the individual elements in greater detail, the overallperformance requirements which the invention preferably meets toaccomplish its unexpected advantages and benefits over the conventionalsystems, are described below.

For example, the structure of the invention preferably generates a beamlanding energy of 500-800 eV and a spot size less than or equal to 5micrometers, with a beam convergence semi-angle at the target of atleast 5 milliradians. Maximum allowable deflection distortion ispreferably limited to less than or equal to 1 mm. The initial beamenergy is preferably greater than 5 kV and the deflection field size ispreferably 30×30 mm., whereas the maximum beam landing angle isadvantageously less than 5 degrees. The magnitude of the electrostaticfield at the extraction grid is preferably less than 5 V/mm and theelectrostatic field above the extraction grid is designed so thatelectrons emitted from the substrate which pass through the gridcontinue to travel away from the substrate as opposed to returning tothe substrate. For the purposes of this description, an electrostaticfield having this characteristic of not returning emitted electrons tothe sample will be referred to as "extractive".

The criteria above are derived from the charging-characteristics oftypical substrate materials, the expected minimum size of substratefeatures to be tested, the desired throughput which necessitates beamcurrent and deflection field size, and the electrostatic field requiredfor successful practice of the induced current test method (ICTM).

The beam landing energy criteria represents a worst case assumption withrespect to the maximum landing energy which can be tolerated withoutsignificant insulator charging. If higher beam landing energies can beused with a particular insulator without causing insulator charging,then this higher energy can be used with improved spot size and beamcurrent.

The expected spot size for a given design is calculated theoretically bymethods well known in the field. The spot size is estimated byroot-mean-square (rms) summation of the contributions from theaberration coefficients, assuming a 5 milliradian beam convergencesemi-angle at the target. It is further assumed that curvature of fieldand astigmatism are corrected by stigmators and dynamic focus coils ascommonly known in electron beam lithography. Deflection field distortiondoes not affect spot size and is in principle completely compensable inthe deflection drive electronics. However, retarding field lenses maysuffer from extreme distortions which may make compensation difficult.Thus, the present invention preferably limits distortion of theretarding field objective lens to less than 1 mm.

An initial primary beam energy of 5 kV is adequate to achieve high gunbrightness. However, higher initial beam energies can be chosen but atthe cost of decreased deflection sensitivity.

Since the induced current test method (ICTM), apart from actual tablemove time (e.g., moving specimen transfer table 30 shown in FIG. 4 froma lock port position 22 for loading a specimen onto the table to aposition for analysis of a specimen), has no throughput penaltyassociated with having a deflection field smaller than the substrate, afield size of 30×30 mm is preferably used and is deemed adequate.

Since secondary electron (SE) emission depends on the angle of incidenceof the primary beam, charging uniformity is improved if the beam landingangle is limited to near normal incidence (e.g., an angle ofsubstantially 90-degrees). In one embodiment, the present inventionpreferably allows up to a 5-degree departure from normal incidence, butof course any limit may be chosen as appropriate and thus the inventionis not limited to a 5-degree departure from normal incidence.

The individual elements of the invention will now be described in moredetail.

Retard/Extract Field Design

The above criteria related to the magnitude of the electrostatic fieldat the extraction grid and the requirement that the electrostatic fieldabove the extraction grid be extractive, arise from the ICTM.

For the preferred embodiment of the ICTM, as described in Golladaymentioned above, a planar extraction grid(s) 505 (shown in FIG. 5, forexample) formed of conductive wire (e.g., formed of wire meshes formedby first and second conductors orthogonally formed to one another, orthe like), is preferably positioned above and parallel to the substratesurface. The extraction grid is biased by known biasing means (e.g.,grid bias means) to determine the electrostatic field seen by emittedelectrons and thereby to control the electron-beam charging process.

The purpose of the grid is to ensure that all nodes to be tested aresubject to the same applied electrostatic field irrespective of theirposition within the deflection field. If the deflection field is smallenough, the extraction field uniformity may be adequate for testingwithout an extraction grid. However, for the application to a field sizeof 30×30 mm a grid is preferably used. The grid is constructed of finewire or the like, so as to be effectively transparent to the primaryelectron probe beam so that the primary beam can penetrate the gridwithout an unacceptable amount of dispersion. The grid is preferablymounted to but electrically isolated from the bottom retard electrode.

The magnitude of the electrostatic field at the grid is preferably lessthan 5 V/mm for several reasons.

First, the grid has a delicate physical construction and thus theelectrostatic forces to which the grid is subjected should be minimized.Secondly, the grid should preferably approximate an equipotentialsurface to minimize any deleterious effects of the fine structure of thegrid on the optics of the retarding lens. The non-uniformities ofpotential associated with the discrete grid wire structure are minimizedwhen the electrostatic field in the vicinity of the grid is minimized.Since the region below the grid is substantially field-free (e.g., freeof any electrostatic field), the field above the grid is preferablyminimized as well.

Furthermore, electrons (e.g., SEs) emitted from the substrate and whichpass through the extraction grid preferably are prevented fromsubsequently returning to the substrate by the electrostatic field abovethe extraction grid. Electrons which return to the sample wouldconstitute a redistribution current. The redistribution current would bedetected by the induced current detector. Redistribution current ispreferably minimized so that the induced current detected isrepresentative only of the charging current injected into or emittedfrom the node under test, i.e., that node whose capacitance is beingmeasured.

Return of emitted electrons may appear unlikely in view of the fact thatthe electrostatic field, which retards the primary beam, acts as anaccelerating field for the emitted SE and back-scattered electrons(BSE). After all, according to the present invention the substrate is athigh negative potential (e.g. -4.4 kV).

However, as described below, it is desirable to operate one or more ofthe retarding electrodes at a potential even more negative than that ofthe substrate. In fact, considering only the primary beam optics,electrode potentials which would return SEs to the substrate providequite acceptable performance. Therefore, a compromise must be made tooptimize the retarding field for both the primary and emitted electrons.

These aspects of the invention are perhaps best illustrated byconsidering their implications for the axial potential distribution inthe retarding electrostatic field. Using nominal values of substratepotential of -4.4 kV, and of the upper column 0 V, several possibledistributions of the potential along the symmetry axis of the retardingfield are illustrated in FIG. 6.

However, the distribution of curve a in FIG. 6 cannot be used because itis not extractive. The lowest energy emitted electrons would bereflected when they encounter equipotentials more negative than the -4.4kV of the substrate, and of these electrons, some (depending on emissionangle) would return to the sample.

Curve b is extractive but has overly strong fields at the grid. Incontrast, Curve c is illustrative of the many possible distributionswhich are acceptable in terms of being extractive and yet havingacceptably weak fields at the extraction grid.

The above discussion has related to the properties of acceptable axialdistributions. Of course, the retarding potential distribution must meetthe above requirements over the entire 30×30 mm deflection field. Theserequirements can be summarized succinctly in terms of the cylindricalcoordinate system (r, z) illustrated in FIG. 7, in which r=0 defines theaxis of symmetry for the magnetic and electrostatic elements, and z=0 atthe surface of the substrate, and the extraction grid is at z_(grid).The electrostatic field above the sample is described by the potentialdistribution V (r, z) . If the maximum deflection is denoted by R_(max),the field is extractive over the entire deflection field if

    V(r,z)≧V(r, z.sub.grid); for z≧z.sub.grid, 0≦r≦R.sub.max.

Potential barriers at r>R_(max) are permissible because few emittedelectrons will encounter these barriers, and most of those emittedelectrons that do encounter the barriers are prevented from reaching thesample by the solid structure of the retard electrode outside thedeflection field area.

The requirement to minimize electrostatic field strength at the grid canbe stated in terms of the derivative of the electrostatic potential asfollows:

    δV(r, z.sub.grid)/δz≦5 V/mm; for 0≦r≦R.sub.max.

The retarding field arrangement according to the invention meets therequirements above and also those relating to the primary beam optics byproviding a plurality of retarding electrodes, whose bias potentials areappropriately chosen as described below for one preferred embodiment.

Except for the bottom electrode, simple cylindrical tubes were chosen asthe basic shape for the retarding electrodes, since detailed analysisshows that electrodes having such a configuration, if properly biased,as described below, are adequate to achieve the given design objectives.The bottom electrode has in addition to the cylindrical portion, aplanar portion with a cutout where the extraction grid is mounted. Thisform and configuration of the electrodes facilitates the manufacturingand assembly of the present invention. However, as is clearly evident,more elaborate electrode configurations can be used, as desired, and itis envisioned that one of ordinary skill in the art taking thisapplication as a whole could easily tailor the configuration of theelectrodes to a plurality of shapes.

Again referring to FIG. 7, which illustrates a cross-section of aplurality of electrodes 703 of the retarding field lens system, the axisof cylindrical symmetry is shown as a vertical dashed line. The bottomelectrode (e.g., V4) corresponds to the extraction grid which is biasedto V_(sub), nominally -4.4 kV. The substrate is assumed to be severalmillimeters below the extraction grid and below the substrate is aplanar electrode also biased to V_(sub). The substrate is thereforebetween two electrodes at the same potential and thus there are noelectrostatic fields present at the substrate.

The top electrode V1 is grounded so that the electrode structure can besimply mounted to the grounded body of the magnetic lens (unreferenced)above. The bottom electrode and extraction grid are biased to the samplepotential V_(sub). For the given geometrical arrangement of electrodes,two free parameters (e.g., the potentials on the interior electrodes, V2and V3, respectively) remain which can be selectively adjusted to obtainthe required extraction field above the grid and to assist in meetingthe criteria for the primary beam.

Appropriate values of V2 and V3 are chosen by considering that the fieldabove the grid be extractive for SEs. For a given electrode geometry andany values of V2 and V3, the potentials throughout the retarding fieldare calculated by standard numerical methods, such as those embodied inthe software package "Litho" which is commercially available, and wellknown in this field.

For each value V2, the minimum value of V3 was determined such that thepotential V (r, z) is extractive for 0≦r≦R_(max). For the electrodegeometry of FIG. 7 and for R_(max) 23 mm (allowing coverage of a 30×30mm deflection field), the results of the analysis are shown in FIG.8(a). For any given value of V2, V3 must be on or above the curve inFIG. 8(a) in order for the field to be extractive over the entiredeflection field.

To further narrow the choices for V2 and V3, the criteria of minimalextraction field strength at the extraction grid was applied. In FIG.8(b), the field strength at the grid is plotted as a function of V2. Foreach value of V2, V3 is given by the curve in FIG. 8(a). The fieldstrength is shown both at the center and at the corner of the deflectionfield. Acceptably weak fields (<5 V/mm) are obtained provided that

    -13.5 kV<V2<-12 kV.

The axial potentials obtained for V2=-12.8 kV and V3=-2.257 kV areillustrated in FIG. 9 in which the potential, V(r, z) for r=0 and r=23mm is plotted. The plots illustrate the fulfillment of the extractiveand weak criteria.

The other components of the retarding field objective lens and thecomplete system with respect to its performance in focusing anddeflecting the primary beam are described below.

Magnetic Lens and Deflection Yokes

Again looking at the retarding field objective lens design shown in FIG.5, the retarding field objective lens system 500 preferably includes acombination of a magnetic lens 500a, upper and lower magnetic deflectionyokes (e.g., coils) 501 in addition to the electrode assembly 502mounted beneath the magnetic lens 500a.

The magnetic lens 500a is conventional. However, to minimize overallsystem length, a large bore design is preferably used so that a largediameter yoke (e.g., coil) can be accommodated within the lower part ofthe lens bore. The meaning of the term "large bore", with regard to thisapplication, will be described below.

The excitation of the magnet lens 500a is adjusted to focus the primarybeam onto the substrate 504. Dynamic focus and stigmator elements, (notillustrated) as are commonly known, are provided in accordance withstandard practice for electron beam lithography columns.

The deflection yokes 501 are conventional and can be of a saddle ortoroidal configuration. The yoke turns are preferably distributedangularly to eliminate four-fold deflection aberrations in accordancewith standard practice. To keep the overall system length short thecoils are positioned partially within the body of the magnetic lens 501.However, the coils are located below the lens gap, because in thatposition the coils can be surrounded by ferrite shielding materialwithout materially disturbing the lens field. The cylindrical ferriteshield is needed to prevent the penetration of the magnetic deflectionfields into the bulk metallic components of the magnetic lens 501, whereeddy currents would be generated which in turn would cause timedependent beam position errors.

The optical properties of a complete system including the magnetic lens,deflection yoke, and retarding field have been calculated with theaforementioned commercially available programs for electron-opticaldesign. With these programs, the influence of the magnetic lensgeometry, the yoke geometry, position, orientation, and deflectionsensitivity on the optical performance can be calculated.

These calculations show that a wide range of yoke geometries andpositions can provide acceptable performance provided there are at leasttwo sets of yokes, an upper yoke, and a lower yoke and that the two setsare in a proper relationship in terms of the deflection sensitivity ofthe upper yoke relative to the lower yoke, and the angular orientationof the upper yoke relative to the lower yoke.

The optimal yoke relationship has the upper and lower yokesapproximately opposing each other, with the lower yoke having higherdeflection sensitivity. This relationship causes the deflected beam tocross the optical axis of the system deep within the retarding field.The deflected beam trajectory within the retarding field is illustratedin FIG. 10. The deflected trajectory of FIG. 10 is critical to achievingthe desires of the system performance and is fundamentally differentfrom that of the conventional system shown in FIG. 2, in which the beamenters the retarding field parallel to the optical axis.

Since, as described above, the optimum yoke configuration has the upperand lower yokes opposing each other, the primary beam is relatively faroff-axis in the lower yoke. The accuracy of the electron-opticalmodeling software used to calculate the lens properties is compromisedif the electron trajectories are off-axis by a distance which isrelatively large compared to the yoke radius. The acceptable minimumyoke radius must be chosen taking this factor into account, and willvary with the field size and beam size required for the application. Inone embodiment, for example, the present invention has utilized a yokeradius approximately six times as large as the beam off-axial distance.

The factors which dictate system design include beam landing angle andthe coma, transverse chromatic, and distortion aberrations. The designobjectives for system performance can be met if the relative strengthsand angular orientations of the upper and lower deflection yokes areoptimally chosen.

For example, if the ratio of magnetic field strength in the lower yoketo the upper yoke is denoted by S, and by the relative orientation oflower to upper yokes by θ, then there exists an operating windowcentered on S=3, θ=-171° which meets the criteria discussed above.

FIG. 11 illustrates how the landing angle and the aberrations change asa function of θ for S=3. An energy spread for the beam of 1.5 ev and a 5mr beam convergence half-angle at the target which is ample to provide abeam current of at least 100 na, are also assumed.

FIG. 12 shows the calculated first and third order optical properties ofthe an embodiment of the retarding field objective lens for S=3.0,θ=-170°.

Induced Current Detector

Another aspect of the present invention is an induced current detector.As current is injected into networks by the electron beam, currents areinduced in a conducting structure capacitively coupled to the networks.The induced current signals are analyzed to determine substrate opensand shorts and to measure network capacitance.

In contrast to the conventional approach which would have the substrateand associated capacitively coupled structure and current amplifier atground potential, for the present invention the conductive structure isat the elevated negative potential required to elevate the samplepotential to achieve the desired beam landing energy. Referring to FIG.5, the induced current amplifier 503 is therefore constructed so that itcan be referenced to the negative high voltage, e.g., -4.4 kV, andthereby provide the negative high voltage for the conductive structure506. The induced current in the structure 506 is detected, amplified,and down-converted to a signal referenced to 0 volts for furtherprocessing.

In implementation, the induced current detector is preferably used inconjunction with the system illustrated in FIG. 3 or FIG. 5.

According to the present invention, a retarding field objective lens anddeflection system is provided to produce a small, high current probe,and means including magnetic deflectors for deflecting the probe over alarge field while maintaining acceptable spot size.

With the invention, a completely non-contact testing method and systemis provided which avoid contact damage and specimen contaminationproblems which are inherent to contact testing methods, and whichincrease the testing speed using the electron beam as opposed to amechanically movable probe. The invention teaches how to design and biasa retarding field electrode assembly so as to provide a retarding fieldwhich is compatible with the practice of the induced current testmethod; that is a retarding field suitable for use either with orwithout a planar extraction grid provided above the substrate beingtested. In either case the retarding field provides a weak andrelatively uniform extraction field for electrons emitted from thesubstrate. The invention includes means for detecting the inducedcurrent signal from a substrate at elevated potential.

While the invention has been described in terms of a two preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is as follows:
 1. An electron optical systemfor irradiating a sample, comprising:an electron source for producing afirst electron beam, wherein said first electron beam has an energylevel above 1 kV; a retarding field objective lens system for receivingand focussing said first energy electron beam to produce a secondfocussed electron beam, wherein said second focussed electron beam hasan energy level below 1 kV; and a magnetic deflector for deflecting saidsecond focussed electron beam to said sample, thereby to expose saidsample to said second focussed electron beam, and simultaneouslymaintaining a predetermined spot size of said second focussed electronbeam, said sample being biased to a predetermined elevated potential,wherein said magnetic deflector includes a plurality of deflectionyokes, wherein a second deflection yoke of said deflection yokes ispositioned closer to said sample than a first deflection yoke of saiddeflection yokes, said second deflection yoke having a higher deflectionsensitivity than that of said first deflection yoke and having adeflection direction substantially opposed to that of said firstdeflection yoke such that a deflected electron beam approaches anoptical axis of said system within a retarding field formed in saidsystem.
 2. An electron optical system according to claim 1, furthercomprising an induced current signal detector for detecting an inducedcurrent signal being produced when said sample is exposed to said secondfocussed electron beam.
 3. An electron optical system according to claim1, wherein said retarding field objective lens system includes means forretarding electrons in said second focussed electron beam directed tosaid sample and for accelerating electrons, emitted by said sample uponbeing irradiated by said second focussed electron beam.
 4. An electronoptical system according to claim 1, wherein said retarding fieldobjective lens system includes a magnetic lens having a predeterminedbore, and first and second magnetic deflection yokes,wherein a relativestrength and a relative orientation between said first and secondmagnetic deflection yokes are selectively adjustable so as to maintain apredetermined electron beam spot size and a predetermined electron beamlanding angle.
 5. An electron optical system for noncontact testing ofan electrical device, comprising:means for biasing said electricaldevice to a predetermined elevated potential; means for receiving afirst electron beam and for producing a second focussed electron beam toexpose said electrical device to said second focussed electron beam, andsimultaneously maintaining a predetermined spot size of said beam, aninduced current signal being produced when said electrical device isexposed to said second focussed electron beam, wherein said firstelectron beam has an energy level above 1 kV and said second focussedelectron beam has an energy level below 1 kV; induced current detectingmeans for detecting said induced current signal to determinecharacteristics of said electrical device; and means for deflecting saidsecond focussed electron beam, comprising a plurality of deflectionyokes, wherein a second deflection yoke of said deflection yokes ispositioned closer to said sample than a first deflection yoke of saiddeflection yokes, said second deflection yoke having a higher deflectionsensitivity than that of said first deflection yoke and having adeflection direction substantially opposed to that of said firstdeflection yoke such that a deflected electron beam approaches anoptical axis of said system within a retarding field formed in saidsystem.
 6. An apparatus for noncontact testing of a sample,comprising:means for biasing said sample to a predetermined elevatedpotential; means for producing a focussed electron beam having an energylevel below 1 kV; means for deflecting said focussed electron beam tosaid sample, thereby to expose said sample to said focussed electronbeam, and simultaneously maintaining a predetermined spot size of saidbeam, an induced current signal being produced when said sample isexposed to said focussed electron beam; and induced current detectingmeans for detecting said induced current signal to determinecharacteristics of said sample, wherein said means for deflectingincludes a plurality of deflection yokes, wherein a second deflectionyoke of said deflection yokes is positioned closer to said sample than afirst deflection yoke of said deflection yokes, said deflection yokehaving a higher deflection sensitivity than that of said firstdeflection yoke and having a deflection direction substantially opposedto that of said first deflection yoke such that a deflected electronbeam approaches an optical axis of said apparatus within a retardingfield formed in said apparatus.
 7. An apparatus according to claim 6,wherein said means for producing includes a magnetic lens having apredetermined bore, and first and second magnetic deflectionyokes,wherein a relative strength and a relative orientation betweensaid first and second magnetic deflection yokes are selectivelyadjustable so as to maintain a predetermined electron beam spot size anda predetermined electron beam landing angle.
 8. An apparatus accordingto claim 7, wherein said electron beam producing means includes meansfor producing said focused electron beam having a predetermined kineticenergy,wherein said means for producing includes means for deceleratingsaid electron beam in a predetermined portion between said magnetic lensand said sample thereby to provide an electron beam having apredetermined landing energy lower than that of said predeterminedkinetic energy.
 9. An apparatus according to claim 8, wherein saiddecelerating means includes means for producing an electrostatic fieldhaving a predetermined axial potential distribution.
 10. An apparatusaccording to claim 6, further comprising means for retarding electronsin said focussed electron beam directed to said sample and foraccelerating electrons, emitted by said sample upon being irradiated bysaid focussed electron beam, in a direction away from said sample. 11.An apparatus according to claim 8, wherein said biasing means biasessaid induced current detecting means to said predetermined elevatedpotential.
 12. An apparatus according to claim 6, wherein said inducedcurrent detecting means includes a current amplifier, said inducedcurrent being induced in a conductive structure one of within saidsample being tested when said sample is irradiated by said focussedelectron beam and external to said sample being tested.
 13. An apparatusaccording to claim 6, wherein said induced current detecting meansincludes a current amplifier, wherein said amplifier is at said elevatedpotential of said sample.
 14. An apparatus according to claim 6, whereinsaid means for retarding includes a magnetic objective lens having apredetermined bore, and first and second magnetic deflectionyokes,wherein a relative strength and a relative orientation betweensaid first and second magnetic deflection yokes are selectivelyadjustable so as to maintain a predetermined electron beam spot size anda predetermined electron beam landing angle.
 15. An apparatus accordingto claim 6, further comprising an extraction grid positioned betweensaid means for deflecting and said sample.
 16. An apparatus according toclaim 15, wherein an electrostatic field at said extraction grid is lessthan 5 V/mm and the electrostatic field above the extraction grid isextractive.
 17. An apparatus according to claim 15, further comprisingmeans for biasing said extraction grid for altering an electrostaticfield of said extraction grid.
 18. An apparatus according to claim 15,wherein an electrostatic field above said extraction grid is extractivefor emitted electrons.
 19. An apparatus according to claim 15, furthercomprising a plurality of electrodes for retarding electrons in saidfocussed electron beam directed to said sample and for acceleratingelectrons, emitted by said sample upon being irradiated by said focussedelectron beam, wherein a first electrode of said plurality of electrodesis at ground potential and wherein potentials of electrodes beneath saidfirst electrode are adjustable such that an electrostatic field abovesaid extraction grid is selectively adjustable.
 20. An apparatusaccording to claim 6, wherein an angle of incidence of said electronbeam is no more than 5 degrees with respect to the normal to a surfaceof the electrical device being irradiated.
 21. An apparatus according toclaim 6, wherein said induced current detecting means comprises meansfor detecting, amplifying and down-converting said induced current to asignal referenced to 0 volts.
 22. An apparatus according to claim 6,wherein said induced current means comprises:a current amplifierconnected to a conductive structure one of within said sample andexternal to said sample, said induced current being referenced to anegative voltage.